Manually Position Vertex Normals?
The only method that I know of is to influence the normals by using the Normal Edit Modifier and it works only if you have two or more objects and not one object with several submeshes. 


In Object Mode, select Object1, go to "Properties Window -> Data"
and enable "Auto Smooth". 
With Object1 still selected, go to "Properties Window -> Modifiers
-> Add Modifier -> Modifiy -> Normal Edit".
Keep "Radial"and under "Target Object" select Object2.
Repeat steps 1-3 for Object2, only with Object1 as target.




Since gif isn't suitable to display the "before" and "after" images, here's a jpeg to illustrate the difference.



I only tested this once with Leadwerks Game Engine and there it seemed to work.